Study On The Microstructure,Mechanical Properties And Corrosion Resistance Of Microalloyed Mg-Li-Al Duplex Magnesium-lithium Alloys

Mg-Li alloy,as the lightest metallic structure material,has a broad development prospect in the field of transportation system and electronic devices due to its excellent properties such as high specific stiffness,high specific strength and excellent electromagnetic shielding performance.However,the low strength and poor corrosion resistance of Mg-Li alloy greatly limit its practical application.Therefore,it is of great practical significance and theoretical value to study the mechanical properties and corrosion resistance of Mg-Li alloys.In this paper,the microstructure,mechanical properties and corrosion resistance of a series of Mg-Li-Al-X(Sm,Gd,Sn,Cu)duplex magnesium-lithium alloys were systematically studied by means of microstructure characterization,mechanical properties and corrosion performance tests,providing theoretical support for the subsequent application and development of high strength and corrosion resistance Mg-Li alloys.Through investigating microstructure and mechanical properties of the as-cast and extruded LA95-xSm(x=0,0.4,0.8)alloys,it demonstrates that the LA95 matrix alloy is composed of ?-Mg,?-Li and a small amount of AlLi/MgLi2Al phases.With the addition of Sm,the ?-Mg matrix is obviously refined.This is attributed to the good crystallographic orientation relationship:(111)?12Sm//(0001)?-Mg,the mismatch was 12.6%(less than 15%),which illustrated that the Al2Sm phase was considered as an effective heterogeneous nucleation substrate of the ?-Mg phase to refine the ?-Mg phase in the LA95 alloys.And the as-extruded LA95-0.4Sm alloy exhibited an optimum ultimate tensile strength of 283 MPa,which was?25%higher than that of the matrix alloy.The main factor influencing the strength of the as-extruded LA95-xSm alloys was attributed to grain refinement strengthening and second phase strengthening induced by the Al2Sm particles.Through investigating microstructure and mechanical properties of the extruded LA83-yX(x=0,0.6,1.2,1.8;X is Gd,Sn,Cu)alloys,it demonstrates that the phase composition of LA83 base alloy is the same as that of LA95 alloy.With the addition of Gd,Sn and Cu elements,a large number of blocky particles Al2Gd phase,spherical particles Mg2Sn phase and fine particles AlCuMg phase were generated in extruded LA83 base alloy,respectively.The addition of Gd induced the dynamic recrystallization changes of ?-Mg and ?-Li in the hot extrusion process,which promoted the equiaxial grain refinement of ?-Mg and ?-Li,and the effect was the most obvious.The addition of Sn only induced the dynamic recrystallization change of the ?-Li phase in the hot extrusion process,which led to the occurrence of isoaxial grain refinement of the ?-Li phase,followed by the effect of grain refinement.The addition of Cu did not trigger the dynamic recrystallization change in the alloy,but through the nailing effect of the precipitated phase AlCuMg particles in the matrix,the grain refinement of ?-Mg phase also occurred to some extent,and the thinning effect was the weakest.Among the extruded LA83-yX(Gd,Sn and Cu)alloys,the extruded LA83-1.2 Gd and LA83-1.2 Sn alloys have the highest strength,reaching?278MPa and?269MPa,respectively.Compared with the matrix alloys,the strength increases?35%and?31%,respectively.The extruded LA83-1.2 Cu alloy has the best plasticity,with the elongation of up to 26.7%.Compared with the matrix alloy,the plasticity is increased by?117%.The improvement in plasticity is mainly due to the combined effect of a sharp decrease in the volume fraction of ?-Mg phase(which is a hard phase)and a sharp increase in the volume fraction of ?-Li phase(which is a softened phase).The results of the research on the corrosion resistance of the extruded LA83-1.2X(Gd,Sn,Cu)alloy show that the surface Volta potentials of the constituent phases in the alloy and their size relationships:AlCuMg(?720 mV)>AlLi(?380 mV)>Al2Gd(?290 mV)?-Li(?175 mV)>Mg2Sn(?100 mV)>?-Mg(?40 mV).The corrosion resistance of extruded LA83-1.2X(Gd,Sn,Cu)alloy is related to the potential differences between the second phase and the matrix phase.Compared with AlLi/?-Mg and ?-Li/?-Mg micro-galvanic couples in the extruded LA83 matrix alloy,the potential difference of the newly formed Al2Gd/?-Mg micro-galvanic couple in the extruded LA83-1.2 Gd alloy is smaller,so the corrosion strength of micro-galvanic couples is weaker.Compared with the matrix alloy,the weight loss rate is reduced by?69.2%.The beneficial effect of the passivation film formed by the compact corrosion products generated by the extrusion LA83-1.2 Sn alloy in the corrosion process was stronger than that of the Mg2Sn/?-Mg micro-galvanic couple corrosion,which resulted in a decrease in the kinetic rate of the total cathode reaction,and the weight loss rate of the alloy was reduced by-61.6%compared with that of the matrix alloy.In addition,the newly formed AlCuMg phase in the extruded LA83-1.2cu alloy has the highest Volta potential,and the Volta potential difference between AlCuMg and?-Mg or ?-Li was the largest(V(AlCuMg/?-Mg)?680mV,V(AlCuMg/?-Li)?545mV),which accelerated the corrosion rate of the alloy,and the weight loss and hydrogen evolution corrosion rate reached 37.9 times and 27.8 times of the extruded LA83 matrix alloy,respectively.